Protein kinases have been the subject of extensive study in the search for new therapeutic agents in various diseases, for example, cancer. Protein kinases are known to mediate intracellular signal transduction by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. There are a number of kinases and pathways through which extracellular and other stimuli cause a variety of cellular responses to occur inside the cell.
Human TTK protein kinase (TTK), also known as tyrosine threonine kinase, dual specificity protein kinase TTK, Monopolar Spindle 1 (Mps1) and Phosphotyrosine-Picked Threonine Kinase (PYT), is a conserved multispecific kinase that is capable of phosphorylating serine, threonine and tyrosine residues when expressed in E. coli (Mills et al., J. Biol. Chem. 22(5): 16000-16006 (1992)). TTK mRNA is not expressed in the majority of physiologically normal tissues in human (Id). TTK mRNA is expressed in some rapidly proliferating tissues, such as testis and thymus, as well as in some tumors (for example, TTK mRNA was not expressed in renal cell carcinoma, was expressed in 50% of breast cancer samples, was expressed in testicular tumors and ovarian cancer samples) (Id). TTK is expressed in some cancer cell lines and tumors relative to normal counterparts (Id.; see also WO 02/068444 A1).
Therefore, agents which inhibit a protein kinase, in particular TTK, have the potential to treat cancer. There is a need for additional agents which can act as protein kinase inhibitors, in particular TTK inhibitors.
In addition, cancer recurrence, drug resistance or metastasis is one of the major challenges in cancer therapies. Cancer patients who responded favorably to the initial anti-cancer therapy often develop drug resistance and secondary tumors that lead to the relapse of the disease. Recent research evidences suggest that the capability of a tumor to grow and propagate is dependent on a small subset of cells within the tumor. These cells are termed tumor-initiating cells (TICs) or cancer stem cells. It is thought that the TICs are responsible for drug resistance, cancer relapse and metastasis. Compounds that can inhibit the growth and survival of these tumor-initiating cells can be used to treat cancer, metastasis or prevent recurrence of cancer. Therefore, a need exists for new compounds that can inhibit the growth and survival of tumor-initiating cells.